I need to know how to replace the hi-limit/freeze sensor. If I unhook this my spa runs fine but no heat is working. If I plug in this sensor my spa shuts down. The wire runs into a plastic pipe which goes into the insultion on right side of tub. How do I replace and where is sensor located?

I believe you are referring to the spa temperature sensor. The wire for the sensor should be in a flex pipe. The flex pipe just butts up to the back of the temp sensor. You should be able to pull that flex pipe back, it may be held on only by the foam insulation and not glued to the sensor nut. Once you pull back the flex you should expose the back side of the sensor. There will be a nut holding the sensor to the wall of the spa. The front of the sensor will either be located in the filter bucket or in the footwell area on the side of the wall.